It is a hard topic to grasp, and there is much bogus information out there, even amongst Photoshop instructors.

The pixels per inch setting only has meaning when considering an actual output size in inches. It is pixels per inch. If you change the ppi setting in Photoshop, as you have done, you can see what size it would print at for the resolution you have given it. Conversely, you can change the output dimensions and see what resolution that would be for that size.

If I had a 2000 x 3000 pixel image, for example, and sized it at 4 x 6 inches, the resolution would be 500 ppi (3000 pixles / 6 inches = 500 pixels per inch). The same image at 8 x 12 inches would be 250 ppi (3000 / 12 = 250), still probably fine for printing at that size.

If I wanted to print the same image at 16 x 24 inches, then the resolution would be only 125 ppi (3000 / 24 = 125). At this point I would consider uprezing the resolution to 248, 300, or 360 ppi, depending on the printer. If I want a higher resolution at that size then I have to generate more pixels. After uprezing the image I could examine it and see if it was still suitable for printing at that size.

Just changing the ppi setting in Photoshop does not up the resolution. You can spend all day changing the ppi and it will not give you any more pixels than you started with. To change the resolution you need to check the 'resample image' box. Then when you change either the inch dimensions or the 'resolution', Photoshop will create more pixels out of the pixels you have there.

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